Process for the preparation of taxanes from 10-deacetylbaccatin III

ABSTRACT

A process for the preparation of taxane derivatives by reacting 10-deacetylbaccatin III protected at the 7-and 1-positions with trichloroacetyl groups with a compound of formula                    
     and subsequent removal of the protective groups and hydrolysis of the oxazolidine ring.

This application is a 371 of PCT/EP00/01471 filed Feb. 23, 2000.

TECHNICAL FIELD

The present invention relates to a process for the preparation oftaxanes from 10-deacetylbaccatin III.

The present invention relates to a process for the preparation oftaxanes from 10-deacetylbaccatin III.

BACKGROUND OF THE INVENTION

Paclitaxel is a known antitumor drug with taxan structure, whoseindustrial preparation is particularly complex.

Paclitaxel was first isolated by extraction from the trunk barks ofTaxus brevifolia, and it is at present synthesized starting from10-deacetylbaccatin III, an intermediate present in the leaves ofdifferent species of taxus, particularly in those of Taxus baccata L.,thereby overcoming the environmental problems connected with theavailability of bark of T. brevifolia.

A number of synthetic methods are reported in literature: U.S. Pat. No.Re. 34,277 (reissue of U.S. Pat. No. 4,924,011) discloses the.semi-synthesis of Paclitaxel starting from 10-deacetylbaccatin IIIprotected at the C-7 hydroxyl group with a trialkylsilyl group, inparticular triethylsilyl, and at the 10-position with an acetyl group.In WO 98/08832, the protection of the C-7 hydroxyl group is carried outusing a trichloroacetyl group. The thus protected baccatin IIIderivative is reacted with acetyl bromide and, subsequently, with thesuitable phenylisoserine derivative to obtain Paclitaxel, followingdeprotection of the hydroxyl groups at 7 and 2′ and benzoylation of theamine.

In WO 93/06094, Paclitaxel is prepared by reacting a beta-lactam-typecompound with 7-triethylsilyl-baccatin III. The desired product isobtained by deprotection in acid medium.

In U.S. Pat. No. 5,476,954, the synthesis of Paclitaxel is carried outstarting from 10-deacetylbaccatin III, protecting the C-7 hydroxyl with2,2,2-trichloroethoxycarbonyl(Troc) and the C-10 hydroxyl with Troc orwith an acetyl group.

It is therefore evident that the critical step for the synthesis ofPaclitaxel is the selective esterification at C-7 with a group easilyand selectively removable. Until now, 7-triethylsilyl-deacetylbaccatinIII has been considered the key intermediate. The yield reported for thederivatization of 10-deacetylbaccatin III to7-triethylsilyl-10-deacetylbaccatin III is about 85%, using 5 to 20 molsof silylating agent. The yield of the subsequent acetylation to give7-triethylsilylbaccatin III is also about 85%.

U.S. Pat. No. 5,621,121 and U.S. Pat. No. 5,637,723 disclose thesynthesis of taxanes, including Paclitaxel, by reacting suitablyprotected baccatin III or 10-deacetylbaccatin III withoxazolidine-5-carboxylic acids bearing at the 2-position a phenyl groupsubstituted with alkoxy groups (U.S. Pat. No. 5,621,121) or withtrihaloalkyl groups, in particular trichloromethyl (U.S. Pat. No.5,637,723), followed by deprotection by opening of the oxazolidine ring.

The protective groups considered particularly suitable comprise silyl,2,2,2-trichloroethoxycarbonyl or 2-(2(trichloromethyl)propoxy)carbonylgroups.

Substantially the same methods can also be used for the preparation ofDocetaxel, another known taxan derivative widely used in clinics.

It has now been found a process for the preparation of taxanes, inparticular Paclitaxel and Docetaxel, which attains higher yields thanthe known methods.

SUMMARY OF THE INVENTION

It has now been found a process for the preparation of taxanes, inparticular Paclitaxel and Docetaxel, which attains a higher yield thanknown methods.

The process of the invention, shown in the following Scheme, comprises:

a) simultaneous protection of the hydroxyl groups at the 7- and10-positions of 10-deacetylbaccatin III with trichloroacetyl groups.

b) subsequent esterification of the hydroxyl at the 13-position byreaction with a compound of formula (VII):

 wherein R is tert.butoxycarbonyl, benzoyl or the residue of a straightor branched aliphatic acid and R₁ is phenyl or a straight or branchedalkyl or alkenyl;

c) removal of the trichloroacetic protective groups;

d) optional selective acetylation of the hydroxyl at the 10-position,for those compounds in which R₂ is acetyl;

e) acid hydrolysis of the oxazolidine ring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the invention differs from those of the prior art in thethat the reaction sequence used provides a simpler route than thoseprocesses cited above and a remarkable improvement in the obtainedyields.

The process of the invention differs from those of the prior art in thatthe reaction sequence used provides a simpler route than the knownprocesses cited above and a remarkable improvement in the obtainedyields.

Step a) is conventionally effected with trichloroacetic anhydride insuitable solvents and in the presence of bases such as pyridine,triethylamine and the like.

The esterification with the oxazolidine-5-carboxylic acid derivative iscarried out in the presence of a condensing agent such asdicyclohexylcarbodiimide or other known reagents, in an anhydrousorganic solvent, preferably aliphatic, aromatic or chlorinatedhydrocarbons, at temperatures ranging from room temperature to theboiling temperature of the solvent.

The resulting oxazolidine ester is then deprotected by removing the 7-and 10-trichloroacetyl groups by treatment with NH₄OH/NH₄Cl in aliphaticalcohols, preferably methanol.

The selective acetylation of the hydroxyl at the 10-position is carriedout with acetic anhydride in the presence of cerium III, scandium orytterbium salts, in a solvent such as tetrahydrofuran, dichloromethane,ethyl acetate, at temperatures ranging from 5 to 40° C.

The treatment with organic or inorganic acids in solvents such asmethanol, ethanol, tetrahydrofuran, at temperatures ranging from about−2 to +2° C., yields the desired taxane derivatives. The use of formicacid in tetrahydrofuran at a temperature of 0° C. is particularlypreferred.

The oxazolidine intermediates are known or can be prepared with knownmethods, by reaction of an isoserine ester with 4-methoxy-benzaldehyde.

The choice of anisic aldehyde proved to be particularly important forthe formation of the oxazolidine, in that oxazolidine acid, contrary tothe methods described in U.S. Pat. Nos. 5,621,121, 5,637,723(Rhône-Poulenc Rorer), and in U.S. Pat. No. 5,821,363 (UpJohn), caneasily be crystallized and adjusted to a 95:5 isomer ratio, which isextremely useful and advantageous for the subsequent step. Furthermore,the oxazolidine carboxylic acid obtainable with anisic aldehyde isparticularly stable during the deprotection of the trichloroacetic esterand the subsequent acetylation step. In these conditions,2,4-dimethoxybenzaldehyde used in U.S. Pat. No. 5,821,363 or chloral orp-trichloromethyl-benzaldehyde as described in U.S. Pat. Nos. 5,621,121and 5,637,723 (Rhône-Poulenc Rorer) are not sufficiently stable.

The process of the invention, in addition to Paclitaxel (R=benzoyl,R₁=phenyl) and Docetaxel (R=tert.butoxycarbonyl, R₁=phenyl), alsoprovides other taxane derivatives efficiently and conveniently.

The compounds of formula IV have never been described before and aretherefore a further object of the invention, as intermediates useful forthe synthesis of taxane derivatives.

The following Examples illustrate the invention in greater detail.

EXAMPLE 1 Preparation of 7,10-bis-trichloroacetyl-10-deacetylbaccatinIII

A solution of 10 g of 10-deacetylbaccatin III (18.4 mmol) in 125 ml ofdry methylene chloride and 42 ml of pyridine is added dropwise with 4.77ml of trichloroacetic anhydride (42.32 mmol). The reaction mixture isstirred for three hours or anyhow until completion of the reaction,checked by TLC on silica gel using a 5:5 n-hexane/ethyl acetate mixtureas eluent. Upon completion of the reaction, 5 ml of methanol are addedto destroy the trichloroacetic anhydride excess, then water. The organicphase is thoroughly washed with HCl (0.1 M solution in water) to removepyridine, whereas the remaining organic phase is dried over MgSO₄ andconcentrated to dryness under vacuum. A pale yellow solid (17 g) isobtained, which upon crystallization from chloroform shows the followingchemical and spectroscopical characteristics:

IR (KBr) 3517, 1771, 1728, 1240, 981, 819, 787, 675 cm⁻¹;

¹H-NMR (200 MHz); δ8.11 (Bz AA′), 7.58 (Bz C), 7.46 (Bz, BB′), 6.50 (s,H-10), 5.72 (m, H-H-2), 5.02 (d, J=8 Hz, H-5), 4.95 (m, H-13), 4.37 (d,J=8 Hz, H-20a), 4.18 (d, J=8 Hz, H-20b), 4.02 (d, J=6 Hz, H-3), 2.32 (s,4-Ac), 2.22 (s, H-18), 1.91 (s, H-19), 1.25 and 1.11 (s, H-16, H-17),m.p.=172-175° C., [α]_(D)−36° (MeOH; C=0.6).

EXAMPLE 2 Preparation of13-(2-(4-methoxyphenyl)-N-benzoyl-4-phenyl-oxazolidyl-)-10-deacetylbaccatinIII

17 g of 7,10-bistrichloroacetyl-10-deacetylbaccatin III are dissolved in250 ml of anhydrous toluene and added under stirring with 12.6 g of2-(4-methoxyphenyl)-N-benzoyl-4-phenyl-oxazolidine-5-carboxylic acid and6 g of DCC. After stirring overnight at 40° C., the reaction mixture isfiltered and concentrated to dryness. The residue is dissolved in 300 mlof methanol/tetrahydrofuran and added with 24 ml of a 2M NH₃ aqueoussolution. After 1.5 hours at room temperature the reaction mixture isconcentrated to small volume under vacuum, then diluted with water andthe whole is extracted with ethyl acetate. The extract is concentratedto dryness and the residue is purified on a silica gel column, elutingthe product with a 1:1 ethyl acetate/petroleum ether mixture, to obtain16.8 g of the title product with m.p. 135° C. and [α]_(D)=−58° (MeOH,C=0.5).

EXAMPLE 3 Preparation of13-(2-(4-methoxyphenyl)-N-benzoyl-4-phenyl-oxazolidyl)-baccatin III

A solution of 13.7 g of the product of example II in 200 ml oftetrahydrofuran is added with 56 ml of a 10% suspension of CeCl₃.7H₂O intetrahydrofuran, followed by 5.5 ml of acetic anhydride. After stirringovernight at room temperature, the reaction mixture is filtered, thefiltrate is treated with methanol and concentrated to small volume; themixture is diluted with H₂O and the product is extracted with ethylacetate, to obtain 12 g (84%) of13-(2-(4-methoxybenzilydene)-N-benzoyl-4-phenyl-oxazolidyl-)-baccatinIII having the following physical and spectroscopical characteristics:

¹-NMR: 8.07 (d, Bz) 7.60-7.19 (m, aromatic), 7.48-6.90 (AA′, BB′,p-OMePh), 6.33 (s, H-10), 5.67 (d, J=5 Hz, H-2), 5.56 (br s, H-3′), 4.93(d, J=8 Hz, H-5), 4.90 (brs, H-2′), 4.45 (m, H-7), 4.28 (d, J=8 Hz,H-20a), 4.16 (d, J=8 Hz, H-20b), 3.82 (s, OMe), 2.27 (s, Ac), 2.08 (s,OAc), 1.66 (s, H-19), 1.29-1.16 (s, H-16, H-17), m.p. 146° C.,[α]_(D)=−62° (MeOH, C=0.8).

EXAMPLE 4 Preparation of Paclitaxel

12 g of 13-(2-(4-methoxyphenyl)-N-benzoyl-4-phenyl-oxazolidyl)-baccatineIII are dissolved in 50 ml of tetrahydrofuran and added at 0° C. with 5ml of formic acid; the reaction mixture is left under stirring at 0° C.for three hours, then diluted with water; formic acid is neutralizedwith KHCO₃ and the suspension is repeatedly extracted with ethylacetate. The ether-acetic extracts are washed with water andconcentrated to small volume. Upon crystallization from the samesolvent, 10.5 g of Paclitaxel are obtained having the samechemical-physical and spectroscopical characteristics as described inliterature.

EXAMPLE 5 Preparation of Docetaxel

17 g of 7,10-bistrichloroacetyl-10-deacetylbaccatin III are dissolved in250 ml of anhydrous toluene and added under stirring with 11.6 g of2-(4-methoxyphenyl)-N-tert.butoxycarbonyl-4-phenyl-oxazolidine-5-carboxylicacid and 6 g of DCC. After stirring overnight at 40° C., the reactionmixture is filtered and concentrated to dryness. The residue isdissolved in 300 ml of methanol/tetrahydrofuran and added with 24 ml ofa 2M NH₃ aqueous solution. After 1.5 hours at room temperature, thereaction mixture is concentrated to small volume under vacuum, thendiluted with water and the whole is extracted with ethyl acetate. Theextract is concentrated to dryness and 10 g of this residue aredissolved in THF and added at 0° C. with 5 ml of formic acid. Thereaction mixture is left under stirring at 0° C. for three hours, thendiluted with water; formic acid is neutralized with KHCO₃, thesuspension is repeatedly with ethyl acetate. The organic extracts arewashed with water and concentrated to small volume. Upon crystallizationfrom the same solvent, 9.2 g of Docetaxel are obtained having the samechemical, physical and spectroscopical characteristics as described inliterature.

What is claimed is:
 1. A compound of Formula (IV)

wherein R is a tert-butoxycarbonyl, benzoyl, or straight or branchedchain alkyl carbonyl group; R₁ is a phenyl or a straight or branchedalkyl or alkenyl group; and R₂ is hydrogen or an acetyl group.
 2. Aprocess for preparing a compound of formula I

wherein R is a tert-butoxycarbonyl, benzoyl, or straight or branchedchain alkyl carbonyl group; R₁ is a phenyl or a straight or branchedalkyl or alkenyl group; and R₂ is hydrogen or an acetyl group comprising(a) simultaneously protecting the C-7 and C-10 hydroxyl groups of10-deacetylbaccatin III with trichloroacetyl groups to provide aprotected 10-deacetylbaccatin III, (b) esterifying the C-13 hydroxylgroup of the protected 10-deacetylbaccatin III with an oxazolidine5-carboxylic acid of formula II

wherein R is a tert-butoxycarbonyl, benzoyl, or straight or branchedchain alkyl carbonyl group; R₁ is a phenyl or a straight or branchedalkyl or alkenyl group to provide a protected C-13 esterified10-deacetylbaccatin III having an oxazolidine ring at the C-13 position;(c) removing the trichloroacetyl groups from the protected C-13esterified 10-deacetylbaccatin III to provide a C-13 esterified10-deacetylbaccatin III; (d) optionally acetylating the C-10 hydroxylgroup of the C-13 esterified 10-deacetylbaccatin III; and (e)hydrolyzing the oxazlodine ring of the protected C-13 esterified10-deacetylbaccatin III in the presence of an acid.
 3. The process ofclaim 2, wherein step (b) is carried out in the presence of a condensingagent and a base.
 4. The process of claim 3, wherein the condensingagent is dicyclohexylcarbodiimide.
 5. The process of claim 4, whereinthe base is pyridine.
 6. The process of claim 2, wherein step (c) iscarried out using NH₄OH/NH₄Cl in an aliphatic solvent.
 7. The process ofclaim 2, wherein step (d) is carried out by reacting the C-13 esterified10-deacetylbaccatin III with acetic anhydride in the presence of acerium III, scandium, or ytterbium salt.
 8. The process of claim 2,wherein step (e) is carried out by reacting the protected C-13esterified 10-deacetylbaccatin III with an organic acid or an inorganicacid in an aliphatic alcohol or tetrahydrofuran.
 9. The process of claim8, wherein the acid is formic acid.
 10. The process of claim 2, whereinR is a benzoyl group, R₁ is a phenyl group, and R₂ is an acetyl group.11. The process of claim 2, wherein R is tert-butoxycarbonyl group, R₁is a phenyl group, and R₂ is a hydrogen.